Research Highlights

Shedding Light on Increasingly Bright Night Skies

PNNL paper examines claims that LEDs are the culprit in brighter nocturnal skies

July 2018

A paper written by PNNL researchers argues that in the U.S., upward radiance was “stable” despite the fact that LEDs increased their share of the national street lighting inventory from 6.6% to 28.3%—amounting to millions of street light upgrades. This demonstrates that these factors are not directly coupled. What’s more, the vast majority of these were 4000 K (i.e., “neutral white”) in color temperature.

When a recent study confirmed the Earth is getting brighter at night, attributing much of the increase to needless waste resulting from LEDs reducing the cost of lighting and hence driving unwarranted increases in its use, PNNL researchers felt compelled to respond. Their paper, "Light at Night—A Delicate Balance," offers an alternative perspective on what we can—and cannot—conclude about new evidence of brightening night skies.

The PNNL paper addresses research, published last fall in the journal Science Advances, which showed increasing levels of upward radiant light, as measured via satellite. On average, the world's lighted areas got 2.2 percent brighter each year between 2012 and 2016, according to the study led by Christopher Kyba of the GFZ German Research Centre for Geosciences. Not unexpectedly, the measured changes in upward radiance were larger in less-affluent countries.

"Perhaps the most important takeaway from the study is the stark evidence that local populations in many places around the world want outdoor lighting to support basic needs of safety and transportation at night," write PNNL researchers Bruce Kinzey and Naomi Miller. To some extent, they add, "this is considered a mark of human progress."

Yet some news stories reporting the study, lamenting the trend, directly implicated LEDs, suggesting their widespread adoption has either increased wasteful lighting practices or failed to prevent them. Unfortunately these stories simultaneously omitted acknowledgement of the corresponding benefits newly lighted areas of the world have gained, many of them in developing countries. LEDs are far more efficient and long-lasting than conventional choices such as metal halide or high-pressure sodium lamps, often saving 50% or more of the energy previously used following a conversion from these older incumbents, and in fact have ushered many previously unlighted areas into the modern world.

As is the case with virtually any new technology, however, there have also been bumps in the road. The fact is that some streetlight conversions have been poorly received and have contributed to broader concerns about light pollution. And the evidence shows that additional light at night can have negative consequences on natural habitats for plants, animals, and insects, by altering mating and feeding habits, for example. But the Kyba et al. study did not directly measure light pollution or light emitted specifically from LED fixtures either. It analyzed near-vertical radiance resulting from the accumulation of all illumination exposed to the night environment, from buildings to parking facilities to sports arenas to roadways. Outside of the rare circumstance, the satellite data cannot attribute increased brightness to individual light sources, or determine whether they are appropriately lighted, or how efficiently they have been lighted, etc., and thus provides no evidence that "The Switch to Outdoor LED Lighting Has Completely Backfired," as one media outlet concluded.

A growing population and rising standards of living, leading to the spread of electrification, are what drive the globally increasing use of light at night rather than a shift to any one technology.

This does not detract from the energy savings achieved with LED lighting, either to date or in the future, the authors argue. Rather, it entirely underscores the need for continuing innovation. Preventing the kinds of concerns raised in these articles will require systems that are able to produce only the precise level of light needed, when and where it's needed, with the additional potential to adapt these outputs over the course of an evening as conditions allow.

"LED technology already offers more capabilities along these lines than any other lighting technology invented to date, in addition to their tremendous energy savings benefits" Kinzey says, adding, "That’s not to say we’re entirely there yet."

The bottom line, according to Kinzey: "if we're going to use light at night—and I think most would say that is a foregone conclusion—continuing to improve the lighting technology and control over its various characteristics, such as distribution, output and spectral content, is critical."